This invention relates to a method of testing the integrity of rock bolts anchored within rock strata or similar materials, and more particularly to testing the integrity of grouted rock bolts by determining the resonant frequencies of selected vibration modes. The method is also applicable to testing the tension of expansion nut rock bolts.
Steel roof bolts or rock bolts are used extensively in underground mining to support the ceilings of shafts and tunnels. The bolts, 2 to 8 feet in length, are anchored in the ceiling by resin grouting or by expansion nuts, thus trying the strata of rock together to create a beam which supports the overburden, using the side walls or pillars as foundations. Roof bolts with a typical diameter of 1 inch are inserted into predrilled holes with a bolt head brazed to one end of the bolt supporting a rectangular plate functioning as a washer. The bolts are held in place by an expansion nut at the top of the bolt or by the newer technique in which the bolt is fully grouted to the inside of the hole with polyester resin. In both cases, the roof bolt binds the rock layers together to form the strata into a beam. In the expansion nut method, tensioning of the bolt applies a compressive stress to the strata, and the friction between the layers as a result of this stress provides the shear stress needed to restrict horizontal movements of the layers, prevent buckling, and give the roof span a load carrying capability. With the fully grouted resin bolt, the resin binds the rock strata together and prevents horizontal movement without tensioning the bolt.
No method is presently defined for checking the joint integrity between the resin grouted bolt and the roof and, therefore, how well the roof bolt is anchored in the ceiling. The older expansion nut system has been checked by applying torque to the bolt which gives a measure of the tension in the bolt. This has no meaning for the grouted bolt because voids in the resin can exist which reduce the effectiveness of the bolt, but a torque could still be applied to the bolt head. Other tension measuring schemes such as the ultrasonic forced vibration technique and a liquid crystal, strain measurement technique are similarly applicable only to expansion nut roof bolts.
A method is needed to test grouted roof bolts or rock bolts to see that they are properly grouted and detect poor roof bolting which could cause roof falls. The present method is applicable to expanison type rock bolts as well.